You reserve them just like all other excursions. Look for the
Castaway Cay day on the
Disney cruise website and scroll down to the cabanas. When you can reserve excursions depends on your
Castaway Club level. Just log onto your account and you'll find out the date you can do that. For a July cruise, a popular month, it isn't likely you'll get one unless you are concierge or Platinum. But don't be discouraged! Log on and see what you find (if you have no kids you could consider the adult beach cabanas), then if you don't get one check back once in a while to see if any have become available. Good luck!